Charge coupled device signal processing circuitry is now being used in conjunction with focal plane arrays of infrared detectors to provide solid state imaging devices as may be seen from U.S. Pat. No. 3,883,437 issued May 13, 1975 to K. Nummedal et al. Charge coupled devices of more general application are shown by way of example in U.S. Pat. No. 3,801,883 issued to J. J. Tiemann on Apr. 2, 1974 and in U.S. Pat. No. 3,819,953 issued to C. M. Puckette et al on June 25, 1974. The basic concept of utilizing potential barriers formed under electrodes to control charge flow in such charge coupled devices has been recognized therein. Furthermore, where such devices are to be used in an imaging capacity, efforts have been made to provide "antiblooming" control circuits of the type illustrated in U.S. Pat. No. 3,863,065 issued on Jan. 28, 1975, to W. F. Kosonocky et al and in U.S. Pat. No. 3,866,067 issued Feb. 11, 1975, to J. F. Amelio. In Kosonocky a bus conductor separated from sensing regions by a potential barrier is utilized to remove excess charge generated in response to an optical overload at one particular location which causes an effect analogous to the blur from an intense light spot in a photographic image which is referred to as "blooming". Amelio deals with the same problem by providing charge transfer between a light sensing element and a charge sink region regulated to control the amount of charge allowed to accumulate in the sensing element. These and other antiblooming circuits, however, operate on a fixed potential barrier control and are directed to only one aspect of the exposure problem, namely the aspect of preventing localized high intensity signal from spilling over into adjacent areas and destroying the resolution of the image.
A different problem to which the present invention is directed is encountered when one records an image in an environment having a high steady state background component throughout the image such as is encountered when recording the infrared tail pipe pattern of an aircraft or missile against a background sky having high uniform infrared radiation therein. In this instance one is not primarily concerned with the excess intensity of the tail pipe radiation in the antiblooming sense, but rather with reducing the overall background or steady state component of signal so as to enhance the contrast ratio of the desired signal.